False positive with mypy

Defining a customFunctions, like this:
```
class CustomFunctions(jmespath.functions.Functions):
@jmespath.functions.signature({'types': ['object']}, {'types': ['array']})
def _func_map_merge(self, obj: str, arg: list[dict[str, str]]) -> list[dict[str, str]]:
result = []
for element in arg:
merged_object = super()._func_merge(obj, element)
result.append(merged_object)
return result
```
Drop the following error

```
error: "_func_merge" undefined in superclass
```

I think it should be some missing information on stubs